The purpose of the proposed research is to examining individual differences in stress susceptibility as a moderating factor in the relationship between environmental stressors and child health. Centering on previously observed associations between stress and illness experience this application advances a new account for the apparent unevenness of stress-related effects on health in childhood. It is proposed that past work on the stress-illness hypothesis has been flawed by a failure to address one component of the classic host-environment-illness triad of epidemiologic investigation. Missing from much of previous research is a systematic consideration of individual variability in children's physiologic and behavioral response to stressors. Based on an emerging body of research examining psychobiologic differences in stress response patterns, the proposal predicts the identification of a subgroup of preschool children with evidence of exaggerated. hyperdynamic responses to environmental challenge- lt is further predicted that children demonstrating excessive responsiveness will react to naturally occurring stressors with disproportionate rates of injuries and respiratory tract infections. Finally, the proposal argues that the elucidation of host response as an important, additional factor in the relationship between stress and child health will have significant implications for the development of preventive strategies in pediatrics and behavioral medicine. A population of 70 preschool children attending two university- affiliated daycare centers will be followed for one year, prospectively ascertaining the incidence and severity of injuries and infectious respiratory illnesses. Environmental stress-- comprising acute and chronic stressors occurring in both the family and daycare settings--will be measured using interviews, questionnaires, and daycare staff checklists at baseline and throughout the study period. Stress response will be assessed using: a) physiologic measures of cardiovascular and neuroendocrine reactivity to laboratory challenge, and b) naturalistic observations of behavioral markers, in the form of aggressive and/or inhibited behavior. It is hypothesized that a significant interaction will be found between environmental stressors and individual stress responses, such that children with both a large number of stressors and a high level of stress responsiveness will have the greatest incidence and severity of changes in health.